Gas turbine engines are generally known in the art and used in a wide range of applications, such as propulsion engines and auxiliary power unit engines for aircraft. In a typical configuration, a turbine section of the gas turbine engine includes a turbine engine component such as a turbine nozzle, etc. A turbine engine component comprises an annular array of stationary airfoils (i.e., vanes or simply “airfoils”) that extend between shroud rings. In the gas turbine engine, hot gases from the combustion chamber are directed against the annular array of airfoils. During transient conditions, such as start-up and shut down of the gas turbine engine, the combustion gas temperature rapidly changes. As the airfoils (relative to the shroud rings) are more exposed to the hot combustion gas, the airfoils respond more quickly to the changes in gas temperature. Thus, when the airfoils are heated faster or hotter than the shroud rings, the airfoils become susceptible to large thermal compressive stresses because the airfoils tend to expand but are constrained by the shroud rings. Similarly, when cooled, a large tensile stress is created across the airfoils that tend to induce contraction.
The cyclic nature of the thermal stresses render the airfoils highly susceptible to low cycle fatigue cracking. Moreover, the differences (if any) between the coefficients of thermal expansion of the airfoil material and the shroud ring material may also cause thermal stresses. Therefore, a conventional bi-cast turbine engine component includes slip joints between an end portion of each airfoil in the annular array and an adjacent shroud ring, in order to accommodate thermal expansion of the airfoils. FIGS. 1 and 2 depict a single airfoil in a portion of the conventional bi-cast turbine engine component. An end portion 36 of the airfoil 24 is slip coupled to an adjacent shroud ring 28 by a slip joint 58. The conventional slip joint 58 is formed by the generally convex sloping side surfaces 66 and 68 of the end portion 36. An opposing end portion 32 of the airfoil is anchored by the opposing shroud ring 26. During operation of the gas turbine engine, when the shroud rings 26 and 28 and airfoils 24 are at the same ambient temperature, the slip joints 58 are closed, in the manner illustrated schematically in FIG. 1. However, when the airfoils 24 are heated to a temperature that is above the temperature of the shroud rings 26 and 28, the airfoils expand radially outwardly relative to the shroud rings. There will be greater thermal expansion of the airfoils 24 relative to the shroud rings. As this occurs, the slip joints 58 open, as shown schematically in FIG. 2. As the slip joints 58 open, the airfoil will expand into a space 164 in the adjacent shroud ring 28. The space is formed in the shroud ring during bi-casting of the turbine engine component (resulting in the bi-cast turbine engine component) as hereinafter described.
The bi-cast method of manufacturing a bi-cast turbine engine component is well known in the art. Generally, when the bi-cast turbine engine component is manufactured, the shroud rings are cast after the airfoils have been individually cast and placed in the annular array of an assembly fixture. Core material is disposed at the end portion of the airfoils and is used to form the slip joints between the end portion of each airfoil in the annular array and the adjacent shroud ring. The airfoil and core material are connected by an adhesive bond. The airfoils are positioned in the annular array with the end portion and opposing end portion of the airfoils at least partially enclosed by a shroud ring pattern comprised of a wax material. The exposed surfaces of the airfoils and the shroud ring patterns are covered with ceramic mold material. After the exposed areas of the airfoil and the shroud ring patterns have been covered with ceramic mold material to make a mold, the shroud ring patterns are removed (by melting of the wax material) to leave shroud ring mold cavities, with the core material enclosed in a shroud ring mold cavity. Once the mold has been formed in this manner, the mold is preheated to about 1800° F. The shroud ring mold cavities are filled with molten metal that is then solidified to form the shroud rings. After the molten metal has solidified, the core material is removed from the shroud ring adjacent the end portion to leave the space around the end portion for thermal expansion of the airfoil relative to the shroud ring.
The airfoil material and core material typically have different coefficients of thermal expansion causing thermal stress during manufacture of the conventional bi-cast turbine engine component (more particularly, during the melting and preheating steps), and the adhesive bond between the core material and the end portion of the airfoils may be broken. More specifically, the core material develops cracks as a result of the thermal expansion mismatch and may separate from the end portion of the airfoils. Therefore, when the shroud ring mold cavity adjacent the end portion is filled with molten metal, the molten metal may fill in the space formerly occupied by the now-separated core material, thereby eliminating the slip joint between the airfoil and the shroud ring. Even if the adhesive bond is not broken and the slip joints are successfully formed, the conventional slip joint accommodates thermal expansion of the airfoils, but not thermal contraction of the airfoils that occurs when the shroud rings are hotter than the airfoils. Therefore, large thermally induced loads in the airfoils and inner and outer shroud rings may result.
Accordingly, it is desirable to provide airfoils configured to form improved slip joints in bi-cast turbine engine components and the turbine engine components including the same. It is also desirable to configure the airfoils such that the airfoils can thermally expand and contract during engine operation. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the present invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.